string_decoder: rewrite implementation

This commit provides a rewrite of StringDecoder that both improves
performance (for non-single-byte encodings) and understandability.

Additionally, StringDecoder instantiation performance has increased
considerably due to inlinability and more efficient encoding name
checking.

PR-URL: https://github.com/nodejs/node/pull/6777
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

benchmark/string_decoder/string-decoder-create.js

@@ -0,0 +1,22 @@
+'use strict';
+const common = require('../common.js');
+const StringDecoder = require('string_decoder').StringDecoder;
+
+const bench = common.createBenchmark(main, {
+  encoding: [
+    'ascii', 'utf8', 'utf-8', 'base64', 'ucs2', 'UTF-8', 'AscII', 'UTF-16LE'
+  ],
+  n: [25e6]
+});
+
+function main(conf) {
+  const encoding = conf.encoding;
+  const n = conf.n | 0;
+
+  bench.start();
+  for (var i = 0; i < n; ++i) {
+    const sd = new StringDecoder(encoding);
+    !!sd.encoding;
+  }
+  bench.end(n);
+}